Hello
Okay here goes....
A day bed is a very European thing to have (and USA) Yes it is a bed, but designed to look elegant so that it can almost be mistaken as a comfortable looking sofa, and blend in with the rest of your furniture so that you don't look like you have an actual bed in the middle of your living room. It's a great place to have a nap, without actually having to 'retire' to the bedroom. And is great for that un expected guest. It's called a day bed, because it's not commonly placed in a 'bedroom' so it's used more liberally - other than at 'bedtime'
Cheers!
Bye